Have you checked his temperature rectally today? Sometimes the rectal thermometer, if not lubricated enough, can cause anal fissures (tiny tears) in his rectum. It heals on its own usually within a day or so.

milk allergy. same thing happened to both kids. doc will make you go off dairy completely if you're nursing, or allementum or nutramegin. if you go with formula, cheapest to buy at target if you have a red card or amazon. also, my insurance covered it the first time around. so its worth the phone call to check. go to pediatrician ASAP.

Like pp said, allergy. My LO is bf also and had bloody streaks then specks of blood in her poop. Ped had me off all allergenic foods; dairy, eggs, nuts, fish, and soy x3wks, then slowly intro each food back week by week. It's so hard to give up dairy and eggs! -it's in everything! And u still have 2wks left. But like pp also said, call ur ped, unless it A LOT of blood, it's not urgent, but prob a good idea to see your ped in the AM tomorrow. GL